2,3,5-Trichloropyridine, 2,3,4,5-tetrachloropyridine and 2,3,5,6-tetrachloropyridine are well known prior art compounds. 2,3,5-Trichloropyridine is a crystalline material melting at 48.degree.-48.5.degree. C.
Tetrachloropyridines are well known prior art compounds. 2,3,4,5-Tetrachloropyridine is a colorless liquid having a boiling point of 116.degree.-119.degree. C. at 6 millimeters of mercury and 2,3,5,6-tetrachloropyridine is a white solid having a melting point of 90.degree.-92.degree. C. Both of these compounds have extremely low solubility in water and from moderate to high solubility in common organic solvents.
2,3,5-Trichloropyridine is useful as an intermediate for preparing various compounds having pesticidal activity, for example, the trichloro compound can be treated with an alkali metal hydroxide employing conventional techniques to prepare 3,5-dichloro-2-pyridinol. The pyridinol can then be reacted with a phosphorochloridate or phosphorochloridothioate to prepare toxicants useful for the control of mite, insect, bacterial and fungal organisms as taught in U.S. Pat. No. 3,244,586.
2,3,5-Trichloropyridine can be prepared by a variety of methods. Sell et al. teach reacting pyridine and phosphorus pentachloride in a sealed tube at 210.degree.-220.degree. C. J. Chem. Soc. 73, 437 (1888). Sell, J. Chem. Soc. 93, 437 (1908) suggest the chlorination of pyridine hydrochloride with chlorine gas at 115.degree.-120.degree. for an extended period of time. In a related process, pyridine hydrochloride is treated with liquid chlorine at 80.degree.-225.degree. C. and an HCl Pressure of above 30 psig as taught in U.S. Pat. No. 3,732,230. Another method using 2-amino-3,5-dichloropyridine as a starting material is taught in British Pat. No. 1,215,387.
Tetrachloropyridines are well known plant growth control agents as taught in U.S. Pat. No. 3,420,833 and other patents. These compounds are prepared by many processes including vapor phase chlorination of pyridine, such as taught in U.S. Pat. No. 3,420,833.
Another method of preparing tetrachloropyridines is taught in Collins et al., J. Chem. Soc. (C) pages 167-174 (1971) wherein pentachloropyridine is reacted with hydrazine hydrate in ethanol and the resulting tetrachloro-4 and/or 2-hydrazinopyridine is oxidized with aqueous copper sulfate or silver oxide in ethanol. This reference also teaches pyrolyzing tetrachloro-4-hydrazinopyridine in sand at 160.degree. C. to yield tetrachloropyridine. In addition, this reference teaches the preparation of 2,3,6-trichloropyridine from the reaction of tetrachloro-4-hydrazinopyridine with cuprous oxide in hot water.
While the above prior art methods are useful in the preparation of 2,3,5-trichloropyridine, 2,3,4,5- and 2,3,5,6-tetrachloropyridine, in small yields on a laboratory scale, these methods are too expensive to be carried out on a commercial scale. Therefore, more practical procedures are continuing to be sought.